Proteins synthesized in cells can be categorized by their individual characteristics into those localized in intracellular organelles, such as nucleus, mitochondria, cytoplasm, etc.; those that function by binding to the cell membrane, such as receptors and channeling molecules; and those that function by being secreted to the cell exterior, such as growth factors and cytokines, etc. In particular, protein molecules bound to the cell membrane are responsible for biologically important functions, such as cellular responses towards growth factors and differentiation factors, inflammatory responses, cell-cell interactions, hormone responses, and so on, and therefore, can be target molecules for diagnostic and therapeutic drugs for various types of disorders.
In recent years, as typified by the genome-project, mass gene-cloning-methods employing random approaches are being conducted, and enormous gene sequence information such as large amounts of ESTS (Expressed Sequence Tags) are accumulated (Matsubara, K. Artificial Organs (1996) 20, 823-827). However, the identification of a protein having a desired function from these ESTS is by no means an easy task, and in order to predict and analyze the function of an encoded-protein from gene sequence information, a great deal of time and efforts are required. Therefore, a method to select, at least up to a certain extent, a gene encoding a protein expected to have a desired function at the stage of random cDNA cloning has been long awaited.
Cloning methods utilizing protein localization were developed as a solution to such problems. For example, proteins secreted to the cell exterior have an amino acid sequence comprising 15 to 30 or so amino acid residues vital for secretion, which is generally termed as a secretion signal sequence or a leader sequence.
Tashiro, K. et al. focused their attention on the features of this secretory protein synthesis and developed a cloning method that specifically selects a gene encoding a secretory protein (Tashiro, K. et al., Science (1993) 261, 300-603). When the signal sequence of proteins that are normally secreted to the cell exterior, for example, interleukin-2 (IL-2) receptor, is deleted, they are unable to express on the cell membrane. If the cDNA encoding the secretion signal sequence is fused, this IL-2 receptor can be re-expressed on the cell membrane as a fusion protein. Since IL-2 receptor fusion protein-expressing cells can be selected by an antibody recognizing the IL-2 receptor, cDNA encoding the protein of which the signal sequence introduced to cells have functioned can be isolated. This method is generally called the SST (Signal Sequence Trap) method as it selectively clones a gene encoding a signal sequence. A cloning method for yeasts has also been developed by basically the same principle (U.S. Pat. No. 5,536,637).
However, even if a gene fragment encoding a protein comprising a signal sequence is obtained by this method, one cannot know whether it is a secretory protein, or whether it is a membrane-bound protein. Also, this method requires the utilization of a cDNA library comprising a 5′ end, but techniques for efficiently constructing a cDNA library that selectively contains a 5′ end are not necessarily easy, versatile techniques.
Recently, Ishihara et al. and Nakauchi et al. reported the TMT (Transmembrane Trap) method, which more selectively clones a gene encoding a membrane-bound protein (Yoshikazu Ichihara and Yoshikazu Kurozawa, Abstracts from the Annual Meeting of the Molecular Biology Society of Japan (1998), No. 3-509-P-533, Nakauchi et al. WO98/03645). The method of Ichihara et al. is based on a principle opposite to the above-mentioned SST method. Namely, the extracellular region of the IL-2 receptor and a protein containing the cell membrane-bound region encoded by cDNA are fused, the IL-2 receptor is expressed on cell membrane surface, and the cells are selected using an antibody against the IL-2 receptor. A model experiment of this method confirmed the expression of fusion molecules between type I or type II membrane-bound proteins, or glycosylphosphatidylinositol (GPI) anchor-type membrane-bound protein and IL-2 receptor on the cell membrane using the anti-IL-2 receptor antibody.
However, when the cDNA library was introduced, proteins not comprising the transmembrane region and membrane-bound region were also obtained within the selected cDNA. In other words, the cloning selectivity of the gene encoding the membrane-bound protein obtained by this TMT method is not necessarily high. This shows, for example, that although all fusion proteins not having the transmembrane region and GPI anchor should be secreted in principle, non-specific agglutinations not owing to the transmembrane region and GPI anchor may also occur on the cell membrane depending on the structures and amino acids compositions of the fusion proteins.
Furthermore, in the case of this TMT method, an epitope recognized by the antibody is expressed in the fusion protein. Therefore, even if fusion proteins expressed in the above manner are non-specifically adsorbed onto the cell membrane, the antibody will recognize and bind to the epitope as long as the epitope is exposed. Also, those molecules on the membrane surface that are on their way to being secreted to the cell exterior are also recognized by the antibody. Therefore, it is desired that the selectivity of membrane-bound protein-expressing cells obtained by this TMT method be further improved.